FIGS. 18 and 19 show a schematic configuration of a helical gear 21Z. FIGS. 18 and 19 depict the manner of the teeth oblique (inclined) at the meshing site of the gear. The degree of obliquity is depicted in an exaggerated manner for the sake of understanding.
Helical gear 21Z includes a hub 22a mounted on a rotation shaft, a disk 22b extending outward in the radial direction of the rotation shaft from hub 22a, and a rim 22c provided at the outer side of disk 22b. Teeth 22d oblique relative to the direction of rotation (R) of gear 21Z is provided at the outer peripheral face of rim 22c. 
Helical gear 21Z has the teeth oblique to the direction of rotation of the gear. Accordingly, the engaging torque is dispersed in a direction crossing the direction of rotation of the gear (thrust direction (S direction in FIG. 19)) to suppress noise with little variation in torque. However, as shown in FIG. 19, dispersion of torque causes helical gear 21Z to incline towards the thrust direction (S direction), whereby vibration and noise increase. Therefore, sufficient suppression of vibration and noise cannot be expected.
The gear disclosed in Japanese Patent Laying-Open Nos. 2009-228741 (PTL 1), 2010-101334 (PTL 2), 2008-303968 (PTL 3), 2005-325865 (PTL 4) and 2005-069401 (PTL 5) have the vibration and noise suppressed by providing a through hole in the disk of the gear and devising the rigidity of the disk.